1. Field of the Invention
The invention relates to an optical semiconductor device and a method of fabricating the same, and more particularly to a waveguide type optical semiconductor device having a function of spot-size conversion and a method of fabricating the same.
2. Description of the Related Art
With the recent development of an optical access system typical of "fiber to the home (FTTH)", a semiconductor laser module used for optical communication is desirably fabricated at a lower cost.
One of major factors for keeping a fabrication cost of a semiconductor laser module high is a packaging cost necessary for optically coupling a laser diode to an optical fiber. Hence, an attention has been recently paid to a spot-size converted semiconductor laser diode which readily accomplishes higher optical coupling between a laser diode and an optical fiber. Herein, a spot-size converted semiconductor laser diode (SSC-LD) is a laser diode which enlarges a spot-size at a plane through which a laser beam leaves, to thereby keep a beam divergence angle small. A smaller beam divergence angle would reduce lights radiated into a free space to thereby ensure a higher optical coupling efficiency for optically coupling a laser diode to an optical fiber. In other words, provision of a semiconductor laser diode with a function of a lens would make it no longer necessary for a semiconductor laser diode to have an optical lens system which was absolutely necessary for a conventional semiconductor laser module. Thus, a semiconductor laser module could be fabricated at lower costs.
In order to enlarge a spot-size at a plane at which laser beams leave a laser diode, it would be necessary to make an optical confinement factor small at the above-mentioned plane in an optical waveguide to thereby enlarge an optical field. Specifically, an optical waveguide is designed to include a spot-size converting portion having a thickness smaller than other portions. A spot-size conversion (SSC) structure like this is useful for all of waveguide type optical semiconductor devices such as a an optical semiconductor modulator, an optical semiconductor amplifier and a waveguide pin photo diode as well as a semiconductor laser diode.
For instance, one of SSC-LDs has been suggested by Y. Tohmori et al. in ELECTRONICS LETTERS, Jun. 22, 1995, Vol. 31, No. 13, pp. 1069-1070 (hereinafter, referred to as first prior art). FIGS. 1A to 1E are cross-sectional views of a laser diode showing respective steps of a method of fabricating a laser diode in accordance with the first prior art.
As illustrated in FIG. 1A, a laser active layer is formed on an InP substrate 401. The laser active layer comprises a first separate confinement hetero-structure (SCH) layer 402, a strained multi-quantum well (MQW) structure 403, and a second SCH layer 404, and each of them are successively epitaxially grown by metal-organic vapor phase epitaxy (hereinafter, referred to simply as MOVPE) growth method.
Then, a SiNx layer 405 is formed on the laser active layer. Then, a portion which would make an SSC portion is etched until the InP substrate 401 appears with the SiNx layer 405 used as a mask. Then, as illustrated in FIG. 1B, an SSC structure comprising a 1.1 .mu.m-bandgap wavelength InGaAsP layer 406 is selectively grown to thereby form a butt-joint.
Then, the SiNx layer 405 is removed, followed by growth of a p-InP clad layer 407 and a p-cap layer 408 all over the product, as illustrated in FIG. 1C.
Then, an SiNx stripe mask 409 is formed partially on the p-cap layer 408, and thereafter the product is etched until a certain depth of the InP substrate 401 with the SiNx stripe mask 409 used as a mask to thereby form a high-mesa structure, as illustrated in FIG. 1D.
Then, the SiNx stripe mask 409 is removed only in the SSC portion, followed by growth of a Fe-doped highly resistive InP layer 410, as illustrated in FIG. 1E. The thus fabricated laser diode has a 300 .mu.m-long laser active layer region and a 300 .mu.m-long SSC region.
In the above-mentioned method of fabricating a laser diode in accordance with the first prior art, it is necessary to repeatedly carry out complicated steps of selective etching and selective re-growth, and it is also necessary to complete a waveguide by forming a butt-joint. Thus, the first prior art has a problem that it is difficult to fabricate a laser diode with a high fabrication yield.
Another example of SSC-LD has been suggested by T. Yamamoto in ELECTRONICS LETTERS, Dec. 7, 1995, Vol. 31, No. 25, pp. 2178-2179 (hereinafter, referred to as second prior art), wherein a multi-quantum well (MQW) structure having different thicknesses and band-gap energies between a laser active layer region and an SSC region is formed by a single selective growth. Hereinbelow is explained the second prior art with reference to FIGS. 2A to 2D.
First, a pair of dielectric masks 502 having a width in the range of tens of micrometers to multi-hundreds of micrometers is formed on an n-InP substrate 501 with the masks 502 being spaced away from each other by 10-20 .mu.m, as illustrated in FIG. 2A.
Then, an n-InP clad layer 503, a strained MQW structure 504, and a p-InP clad layer 505 are selectively grown on the n-InP substrate 501 by MOVPE, as illustrated in FIG. 2B. In this selective growth of the layers 503, 505 and the structure 504, enhancement in a growth rate and increase in an In incorporation rate occur in a region sandwiched between the masks 502 due to vapor phase lateral diffusion of source materials. As a result, a thickness of MQW is enhanced and further a band-gap wavelength is made longer in the region sandwiched between the masks 502 in comparison with other region not sandwiched between the masks 502. Accordingly, the region sandwiched between the masks 502 makes a laser active layer, and the other region not sandwiched between the masks 502 makes an SSC region.
Then, after removal of the dielectric masks 502, a dielectric stripe mask 506 is formed over the selectively grown layers. Thereafter, the product is mesa-etched so that the laser active layer has a width of 1.2 .mu.m, as illustrated in FIG. 2C.
Then, a p-InP current block layer 507 and an n-InP current block layer 508 are grown all over the product. Then, after removal of the dielectric stripe mask 506, a p-InP second clad layer 509 and a cap layer 510 are grown over the n-InP current block layer 508, as illustrated in FIG. 2D. The thus formed laser active layer region is 300 .mu.m long, and the SSC region is 200 .mu.m long.
Still another example of a laser diode has been suggested by M. Wada et al. in ELECTRONICS LETTERS, Nov. 23, 1995, Vol. 31, No. 24, pp. 2102-2104. There has been suggested laser diodes monolithically integrated with spot-size converters operating at 1.3 .mu.m and having an almost circular far-field pattern and a -1.3 dB butt-coupling-loss-to-fiber with wide alignment tolerance. However, the overall device length is 450 .mu.m.
In the above-mentioned first and second prior art, the SSC regions do not have an optical gain, because they are formed merely for enlarging a spot of laser oscillation lights. Accordingly, the first and second prior art are inferior to an ordinary semiconductor laser diode having no SSC region with respect to increasing of a threshold current and degrading performance at high temperature, because the SSC region causes optical losses.
In addition, a device yield per a unit area or per a wafer would be reduced in the above-mentioned conventional SSC-LDs, because they have to be fabricated longer by a length of the SSC region. Specifically, the laser diode in accordance with the first prior art includes the 300 .mu.m long laser active layer region and the 300 .mu.m long SSC region, and hence is totally 600 .mu.m long. The laser diode in accordance with the second prior art includes the 300 .mu.m long laser active layer region and the 200 .mu.m long SSC region, and hence is totally 500 .mu.m long. An ordinary laser diode having no SSC region is 300 .mu.m long. Thus, a yield per a wafer for fabricating devices is reduced by about 40-50% in the first and second prior art in comparison with the conventional laser diodes.
Moreover, the first prior art requires to carry out complicated fabrication steps of repeated selective etching of semiconductor layers and selective re-growth. In addition, since a butt-joint having a problem in repeatability is introduced into a waveguide, the first prior art has a problem in controllability and repeatability, resulting in difficulty in fabricating a semiconductor diode with a high yield and with high repeatability.
Since the second prior art employs mesa-etching/re-growth steps of semiconductor layers for forming an optical waveguide, the second prior art has the same problem as that of the first prior art. Namely, the second prior art has a problem in controllability and repeatability, resulting in difficulty in fabricating a semiconductor diode with a high yield and with high repeatability.